The present invention relates generally to assemblies of ceramic fibers, and, more specifically, to assemblies of piezoelectric materials and the uses therefor.
Piezoelectric materials are materials which have an electrical response to being deformed or stressed, and conversely, deform or xe2x80x9cactuatexe2x80x9d in response to an applied voltage. The ability of piezoelectric material to translate between electrical signals and physical deformation renders them useful in various applications. For example, the inventors have identified applications in which the piezoelectric materials are used for energy harvesting, actuating, sensing, transmitting/receiving waves, and combinations thereof. These applications, however, often require the piezoelectric material to be formed into a fiber composite having a relatively large active surface area compared to traditional fiber composites. Traditional fiber composites tend to be limited in size due to the low strength properties of commonly used piezoelectric materials, such as PZT (lead zirconium titanate).
Prior art approaches for overcoming the weakness of piezoelectric materials to produce a fiber composite having a relatively large surface area have been met with limited success. For example, a common approach for providing fiber composites having a relatively large area involves a diced and filled piezoelectric substrate. More specifically, the piezoelectric substrate is diced by forming a number of perpendicular channels through it. Next, the channels are back filled with a plastic or other high-strength material. Although this approach strengthens the piezoelectric substrate and allows for large active surfaces, it suffers from a number of significant shortcomings, one of the most significant being the occurrence of the Lamb wave mode of vibration. The Lamb wave mode of vibration is created by the lateral motion of the various channels which induces an overall resonance in the fiber composite.
The inventors have developed another approach to prepare fiber composites which involves forming a xe2x80x9cfiber ropexe2x80x9d of piezoelectric material in fiber form. More specifically, individual fibers of piezoelectric material are prepared using known processes such as those described in U.S. Pat. No. 5,827,797 (which has a common assignee to the present application), incorporated herein by reference. A bundle of these fibers then is braided or otherwise bound together and sintered. After sintering, an epoxy or other suitable matrix material can be added to the sintered bundle to impart strength. At this point, individual fiber composites can be sectioned from the fiber rope. Each fiber composite is a cross section of the fiber rope and has two planar, parallel opposing sides or active surfaces. The fibers are substantially normal to the active surfaces and thus their poling direction is normal to the active surface. Having the piezoelectric poling direction substantially normal to the active surface is a desirable property of a fiber composite.
The use of fiber avoids the occurrence of the Lamb wave mode of vibration since fibers tend to have narrow diameters, and, thus, their radial secondary poling direction is minimal (see, e.g., U.S. Pat. No. 5,869,189, incorporated herein by reference). In other words, the radial movement of fibers tends to be very littlexe2x80x94most of their poling direction is along their length. Additionally, the random diameter and packing of fibers also tends to minimize harmonics.
Although the use of fiber rope avoids the Lamb wave mode of vibration, fiber ropes are nevertheless faced with other significant shortcomings. Perhaps the most significant shortcoming is their limitation in size. More specifically, it has been found that fiber ropes greater than about 1xc2xc in diameter are difficult to produce due to the lack of control over fiber consistency. More specifically, the fibers toward the perimeter of the rope tend to be more tightly packed than the fibers toward the center. This condition worsens as the diameter of the rope increases.
Therefore, a need exists for a piezoelectric fiber composite which has a large active surface but which is not susceptible to Lamb wave mode harmonics and is not limited in area. The present invention fulfills this need among others.
The present invention provides for a piezoelectric fiber composite which has a large active surface and a high degree of fiber consistency. The fiber composite of the present invention avoids the aforementioned problems by using small subassemblies or thin layers of fiber in which fiber consistency can be controlled to form a large fiber assembly having sufficient overall fiber consistency. In other words, the inventors have recognized that the most effective way of controlling the fiber consistency of an entire assembly is by controlling the fiber consistency of its smaller components. After the large fiber assembly is formed, thin fiber composites having opposing (active) surfaces can be sectioned.
By sectioning the fiber composites from a large assembly of consistently arranged fibers, the present invention overcomes the various problems in the prior art. More specifically, since fibers are being used, issues with respect to Lamb wave mode harmonics are avoided as discussed above with respect to the fiber rope. Furthermore, since the fiber assembly comprises the conglomeration of a number of smaller components in which fiber consistency can be readily maintained, the size of the fiber assembly is not limited by fiber consistency. The individual fiber composites are sectioned off from the fiber assembly at the desired thickness and thus have an active surface as large as any side of the fiber assembly.
One aspect of the present invention is a method of preparing a fiber assembly having a large cross section and fiber consistency. In a preferred embodiment, the method comprises: (a) providing a plurality of layers, each layer comprising sintered fibers of piezoelectric material aligned substantially parallel; (b) laminating the plurality of layers; and (c) applying a matrix material to the laminated layers to affix the layers and form a fiber assembly. Having a large assemblage of fibers allows cross sections of the assemblage to be removed as fiber composites. Accordingly, in a preferred embodiment, the method further comprises sectioning a portion from the fiber assembly wherein the portion has two opposing surfaces and contains fibers that are substantially normal to the opposing surfaces. The method also preferably comprises applying an electrode to each opposing surface.
Another aspect of the present invention is the product made from the process described above. In a preferred embodiment, the product comprises a fiber composite comprising: (a) two opposing surfaces, wherein each opposing surface has an area greater than about 1.5 in2; (b) a plurality of piezoelectric fibers wherein the variation of fiber concentration/cm3 of fiber composite throughout the fiber composite does not exceed 20% of the overall fiber concentration of the fiber composite; and (c) a matrix material binding the fibers.
Another aspect of the present invention is the use of the piezoelectric fiber composite described above. In a preferred embodiment, the fiber composite is used for energy harvesting, sensing, wave transmitting/receiving and combinations thereof.